In a field of semiconductor device manufacturing system, devices that scan a semiconductor wafer with a charged particle beam to detect secondary electron signals or reflected electron signals from the semiconductor wafer are essential. As this kind of devices, there is a dimension measuring SEM that measures dimensions of patterns formed on the wafer, or an inspection SEM that checks for pattern defects, both by using signal waveforms representing the shapes of the patterns.
On the semiconductor wafer, the object to be measured and inspected, a plurality of rectangular chips having circuit patterns written thereon are formed in an substantially entire wafer surface. In semiconductor device fabricating workshops of recent years, there are growing calls for a capability to make circuit patterns on the wafer up to as near a wafer periphery as possible for efficient use of the surface area of the wafer, i.e., to obtain as many semiconductor chips from a single wafer as possible. For an increased number of semiconductor chips to be produced from one wafer, a move is already under way to expand the surface area of the wafer itself or increase its diameter.
However, since the dimension measuring SEMs are manufactured in conformity with the diameter of the wafer, if the wafer diameter is increased, a new SEM that matches the increased wafer diameter needs to be introduced, which constitutes a heavy burden in terms of facility investment. For improved return on investment in the dimension measuring SEM, it is desired that a dimension measuring SEM be made available that can deal with wafers of different diameters, such as 200 mm and 300 mm, at the same time. In other words, there is a growing need for a charged particle beam evaluation device capable of evaluating the wafer up to as close to its periphery and even wafers of different diameters.
When dimensions of a pattern formed on the wafer are measured using a charged particle beam or when an inspection is made to detect defects and foreign substances, if there are distortions in electric potential distributions in areas radiated with the charged particle beam, the charged particle beam deflects because of a gradient of electric potential. As a result, the charged particle beam strikes areas displaced from where it is supposed to hit to make measurements or inspections, making the inspection on the target areas impossible. This is generally called a position deviation. It also changes the angle of incidence of the charged particle beam, which in turn results in the pattern signal from the wafer (e.g., image) getting distorted.
This problem becomes prominent as the measuring position or inspection position comes close to the outer periphery of the wafer. This may be explained as follows. In central areas of the wafer, the electric potential distribution surrounding the position where the charged particle beam is radiated is considered symmetric with respect to the beam-applied position, whereas in peripheral areas of the wafer the potential distribution surrounding the beam-applied position is considered asymmetric with respect to the beam-struck position, causing the position deviation.
This problem becomes more serious as the wafer diameter increases. This is because the increased wafer diameter reduces the curvature of the outer circumference of the wafer, allowing device chips to be formed in areas up to near the outer edge of the wafer. This leads to a demand gaining momentum that the wafer be able to be inspected up to peripheral areas much closer to the outer edge of the wafer than ever.
However, as described earlier, the positional deviation of the charged particle beam becomes large as the area of interest gets closer to the outer edge. So, in a large-diameter wafer, areas where measurements and inspections cannot be made unavoidably remain near the outer edge.
Under these circumstances, efforts are being made to develop a technique that prevents distortions in the electric potential distribution near the periphery of the wafer and eliminates the positional deviations that would otherwise be caused by the deflection of the charged electron beam during measurement or inspection (see Patent Literatures 1 and 2).